Article Quantifying Impacts of National-Scale Afforestation on Carbon Budgets in South Korea from 1961 to 2014 Moonil Kim 1,2, Florian Kraxner 2 , Yowhan Son 3, Seong Woo Jeon 3, Anatoly Shvidenko 2, Dmitry Schepaschenko 2 , Bo-Young Ham 1, Chul-Hee Lim 4 , Cholho Song 3 , Mina Hong 3 and Woo-Kyun Lee 3,* 1 Environmental GIS/RS Center, Korea University, Seoul 02841, Korea 2 Ecosystem Services and Management Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, A-2361 Laxenburg, Austria 3 Department of Environmental Science and Ecological Engineering, Korea University, Seoul 02481, Korea 4 Institute of Life Science and Natural Resources, Korea University, Seoul 02481, Korea * Correspondence: [email protected]; Tel.: +82-02-3290-3016 Received: 31 May 2019; Accepted: 9 July 2019; Published: 11 July 2019 Abstract: Forests play an important role in regulating the carbon (C) cycle. The main objective of this study was to quantify the effects of South Korean national reforestation programs on carbon budgets. We estimated the changes in C stocks and annual C sequestration in the years 1961–2014 using Korea-specific models, a forest cover map (FCM), national forest inventory (NFI) data, and climate data. Furthermore, we examined the differences in C budgets between Cool forests (forests at elevations above 700 m) and forests in lower-altitude areas. Simulations including the effects of climate conditions on forest dynamics showed that the C stocks of the total forest area increased from 6.65 Tg C in 1961 to 476.21 Tg C in 2014. The model developed here showed a high degree of spatiotemporal reliability. The mean C stocks of the Cool forests and other forests increased from 1 1 4.03 and 0.43 Mg C ha− , respectively, to 102.43 and 73.76 Mg C ha− at a rate of 1.82 and 1.36 Mg 1 1 C ha− yr− during the same period. These results imply that, although the total Cool forest area of South Korea occupied only about 12.3% (772,788 ha) of the total forest area, the Cool forests play important roles in C balances and forest ecosystems in South Korea. Annual C sequestration totals are projected to decrease at a low rate in the near future because the overall growth rate of a mature forest decreases as the stand ages. Our results quantified forest C dynamics in South Korean forests before and after national reforestation programs. Furthermore, our results can help in development of regional and national forest management strategies to allow for sustainable development of society and to cope with climate change in South Korea. Keywords: afforestation; national forest inventory; forest carbon stock; carbon sequestration; Cool forest 1. Introduction The success of climate change mitigation and adaptation and implementing sustainable development goals depends on a diverse array of ecosystem services provided by forests. Over the coming century, these and other forest-derived ecosystem services will need to be sustained to meet the increasing demands of a growing human population [1]. The difficulty of this task is exacerbated by the impacts of anthropogenic climate change [2]. In addition, deforestation is a major driver of both climate change and biodiversity loss [3]. The various worldwide problems and damage caused by deforestation and climate change have been well-documented. Therefore, many countries, including India [4], China [5], New Zealand [6], Finland [7], the UK [8], and South Korea [9], Forests 2019, 10, 579; doi:10.3390/f10070579 www.mdpi.com/journal/forests Forests 2019, 10, 579 2 of 18 have formulated and implemented a number of policies and programs aimed at forest conservation, afforestation, and reforestation. South Korea is a rare, exemplary model of full-scale reforestation [10]. After the Korean War, almost half of its forestland was destroyed. Since 1973, following this period of serious deforestation, the South Korean government has implemented national plantation programs to promote forest recovery. After about 40 years of these efforts, South Korean forests have successfully recovered, 3 1 3 1 and the stocking stem volume increased from 8.2 m ha− in 1954 to 150.0 m ha− in 2015 [11]. Furthermore, this success has spread to several different sectors with positive outcomes in the realms of land restoration, flood prevention, biodiversity recovery, and increases in water supply and recreational forests [12–15]. One of the major management issues that the forest sector in South Korea is faced with is understanding the impact of climate change and optimizing the use of forest resources [16]. There are numerous impacts of climate change associated with social, environmental, and economic problems, and a number of countries are creating mitigation and adaptation plans for climate change [17]. Carbon storage and sequestration are essential attributes of forests and important components of mitigation and adaptation strategies. However, this in turn indicates that forests are also subject to impacts of climate change [18,19]. Studies on forest carbon (C) stocks and balances in South Korea over the past few decades have been conducted for many years. Previous research based on the national forest inventory (NFI) and theoretical models has shown that the biomass C stocks of South Korean forests increased gradually in recent decades [9,15,20–24]. Lee et al. [24] estimated the forest carbon storage in the period of 1990–1997 as a part of estimating the changes in annual CO2 fluxes in Korea. Choi et al. [15] analyzed the temporal variations in forest biomass, forest carbon density, and forest carbon uptake rate in the 1 1 period of 1954–2000 and reported that Korean forests had a higher carbon uptake rate (Mg C ha− yr− ) in the late 1990s than in earlier years. It was concluded that this high carbon uptake rate resulted from the high growth rate of young trees present as a consequence of the 30 year reforestation and forest management program. However, there were some limitations to that study, such as (1) a relatively short simulation period of not more than two decades, (2) the large spatial scale of the simulation units, (3) insufficient spatial validation, and (4) the lack of an annual climate impact on forest stand dynamics. In addition, Cool forests, defined at the 2018 International Boreal Forest Research Association Conference as forests at elevations above 700 m, have recently needed more careful monitoring and assessment than other forests. This is because Cool forests have been assessed as highly sensitive and vulnerable to climate change in South Korea and worldwide [18,25–28]. Therefore, comparison of C dynamics between the Cool forests and other forests through systematic observation and spatial-temporal modeling is required. The primary objective of this study was to estimate C stocks and their changes in South Korean forests, including climate impacts on forest growth during the post-war period (1961–2014) using a Korea-specific dynamic model. To estimate the spatial distribution of forest C stocks, we constructed a spatial data set for Korean forests using the fifth NFI and the fifth Forest Cover Map (FCM). The model was validated by comparing estimated stem volumes with the observational data in the fifth and the sixth NFI on multiple spatial scales. Finally, we compared C stocks and annual C sequestration of Cool forests with those of other forests in South Korea. 2. Materials and Methods 2.1. Spatial Data for South Korean Forests 2.1.1. Study Area South Korea is a peninsula located in mid-latitude East Asia and has a temperate climate. Annual average precipitation is 1000–1800 mm, and 50–60% of the precipitation is concentrated in the summer (June to August) (Figure1a). The average annual temperature is 10–15 ◦C. More than 60% of the country is mountainous, and the terrain is high in the east and low in the west (Figure1b). Forests 2019, 10, x FOR PEER REVIEW 3 of 18 Most of the forests in South Korea are located in mountainous areas. Forests cover about 63.7% (6,369,000 ha) of the total land area of South Korea, and evergreen forests (mainly Pinus densiflora), deciduous broad-leaved forests (mainly Quercus spp.), and mixed forests made up approximately 40.5%, 27%, and 29.3%, respectively, of the total forest area in 2017 [11]. Cool forests were defined at the 2018 International Boreal Forest Research Association (IBFRA) Conference. Boreal and mountain forests develop over many thousands of years in regions with cold climates. They make up more than one-third of global forests, forming the largest terrestrial Forestsvegetation2019, 10 ecosystem., 579 They are found from the circumpolar belt in the northern hemisphere3 of 18to high-elevation forests spread over the entire planet. Forests on permafrost show many similarities with those in boreal and high mountain ecozones—especially with respect to species and growth Mostpatterns of theand forestsin response in South to climate Korea exposure. are located The in 2018 mountainous IBFRA defined areas. a Cool Forests forest cover in the about temperate 63.7% (6,369,000zone as a ha)forest of at the an total elevation land area above of South700 m. Korea, When and this evergreendefinition forestsis applied, (mainly the Pinustotal Cool densiflora forest), deciduousarea in South broad-leaved Korea is an forestsestimated (mainly 772,788Quercus ha basespp.),d on andhigh mixed spatial forests resolution made (30 up m approximately× 30 m) digital 40.5%,elevation 27%, model and 29.3%,data (Figure respectively, 1c). of the total forest area in 2017 [11]. FigureFigure 1.1.( a()a Study) Study area area and and its ( bits) elevation (b) elevation map and map (c) forestand ( areac) forest map. area (d) Annual map. ( meand) Annual temperature mean (left)temperature and annual (left) precipitation and annual (right) precipitation for South (right Korean) for forests.